Ocean energy in the form of waves, currents, tides, and thermal and salinity gradients can provide an abundant supply of clean and renewable energy. Similarly, wind power may provide renewable energy. However, constructing of safe, efficient, affordable and durable system for converting fluid flow to usable energy remains challenging.
When a fluid flow encounters a front surface of an object, vortices are formed behind the object. This phenomenon is called vortex shedding. The vortices are formed at a vortex formation frequency affected by such variables as the geometry of the object, the speed of the fluid flow, and the properties of the fluid. The vortex formation frequency is known to affect the motion of the object.
According to earlier approaches, vortex shedding phenomena were typically minimized in order to decrease noise and damage to the energy conversion systems. More recent developments, however, have indicated that vortex shedding may be used for increasing energy generation from a fluid flow. For example, U.S. Pat. No. 7,208,845 discloses an electrical power generating system including a vibrating assembly which as a vortex shedding device that sheds vortices in response to fluid flow across vibrating assembly, and generator that generates electrical power in response to vibration of the vibrating assembly. The vortex shedding device sheds the vortices at a frequency which is substantially equal to a resonant frequency of the vibrating assembly.
In a paper titled “VIVACE Vortex Induced Vibration Aquatic Clean Energy: A New Concept in Generation of Clean and Renewable Energy From Fluid Flow”; to Michael M. Bernitsas et. al. (published in Journal of Offshore Mechanics and Arctic Engineering NOVEMBER 2008, Vol. 130/041101-1), the authors discloses a vortex induced vibration aquatic clean energy (VIVACE) which converts ocean/river current hydrokinetic energy to a usable form of energy such as electricity using vortex-induced vibration (VIV) successfully and efficiently. VIVACE is based on the idea of maximizing rather than spoiling vortex shedding, and exploiting rather than suppressing VIV. It introduces optimal damping for energy conversion while maintaining VIV over a broad range of vortex shedding synchronization. VIV occurs over very broad ranges of Reynolds (Re) number.
Patent publications to M. Bernitsas et. al include US 2009/0250129 disclosing fluid motion energy converter for e.g. power plants having power device for converting motion of movable element into usable energy wherein roughness is added to the surface of a bluff body in a relative motion with respect to a fluid. US 2009/0114002 discloses a system for reducing vortex induced forces on bluff structure arranged in fluid, has bluff structure with several rough zones formed on surface in staggered orientation, of specific height with respect to linear dimension. US 2009/discloses a vortex induced force enhancing system for harnessing of clean and renewable energy, having roughness zone defining roughness height extending above surface of zone that is less than or equal to specific percentage of linear body dimension. U.S. Pat. No. 7,493,759 and US Patent Publication 2008/0295509 disclose a fluid motion energy converter having a power device for converting vortex induced motion of a movable element into usable energy.
General Description
The techniques disclosed in the above-described art are based, in part, on vortex induced vibration (VIV), also called “lock-on” or “synchronization”. A “lock-on” or “synchronization” occurs when an oscillator's oscillation frequency (f) and vortex formation frequency (fV) are close to the natural frequency (fN) of the body within a regime of large-amplitude vibration. When lock-on occurs, the amplitude of the oscillation increases, enabling an increased yield of usable energy from the conversion of motion into energy.
In the above-described art, in order to attain lock-on, vortex shedding is affected in a so-called passive fashion. This is achieved, for example, by shaping the object (or part of the object) facing the fluid flow in a manner that causes the vortex formation frequency to reach a desired value. As will be described below, in the detailed description, the inventor has found that passively affecting vortex shedding has some limitations. Namely, lock-on is a natural phenomenon that requires certain flow conditions, (e.g. a flow velocity and/or Reynolds number need to be within specific ranges) in order to occur. The use of such techniques is therefore advantageous only when the fluid flow satisfies such conditions. If such conditions are not satisfied, passively affecting vortex shedding does not cause the occurrence of lock on.
There is a need for a technique which forces the occurrence of a so-called “artificial lock on” for a variety of flow conditions, not necessarily those satisfying the natural lock on conditions. In other words, there is a need for technique for achieving the lock on effect while being independent or at least weakly dependent (flexible) on the fluid flow conditions such as for example a flow velocity and/or Reynolds number.
The present invention solves the above problem by providing a technique for actively controlling flow conditions over a surface of an oscillator in a predetermined manner, in order to affect vortex shedding over an oscillator and thereby increase oscillation amplitude of the oscillator. According to some aspects of the present invention, the active control is performed by affecting promoting time-dependent flow separation over the oscillator's surface. Alternatively or additionally, the active control may utilize time-dependent promotion of flow attachment to the oscillator's surface (i.e. reduction of flow separation over the oscillator's surface).
Therefore, an aspect of some embodiments of the present invention relates to a device for use in extracting energy from an incoming fluid flow. The device includes an oscillator assembly, and operative flow affecting unit, and a control unit. The oscillator assembly is mounted on a base, and includes a main body for exposing to an incoming fluid flow, and a joining element attached to the main body and configured for anchoring the main body to the base and enabling oscillation of the main body with respect to the base. The operative flow affecting unit includes at least one flow interacting element located in at least one location respectively on a surface of the main body, the operative flow affecting unit being configured and controllably operable for affecting a separation of streams of the fluid flowing over the surface of the main body. The control unit is in communication with the operative flow affecting unit, and is configured and operable for activating and deactivating of each of the at least one flow interacting element of the operative flow affecting unit according to a certain time pattern, the time pattern being selected such that interaction between the flow interacting element and fluid streams creates vortices in the fluid streams at a selected vortex formation frequency causing an increase in oscillation of the main body, thereby enabling conversion of motion from the oscillation into useful energy.
In a variant, the operative flow affecting unit is configured such that the at least one flow interacting element is operable for increasing separation between the main body's surface and the streams of fluid flowing over the main body's surface. Additionally or alternatively, the operative flow affecting unit is configured such that the at least one flow interacting element is operable for increasing attachment between the main body's surface and the streams of fluid flowing over the main body's surface.
In a further variant, the joining element is a spring. In yet a further variant, the joining element is a stalk joined to a pivot in the base, the stalk being rotatable at least in one plane with respect to the base, such that the oscillator assembly is a pendulum capable of oscillating about said pivot.
Optionally, the control unit is configured and operable to provide the vortex formation frequency approaching a natural frequency of the oscillator.
In a variant, the certain time pattern according to which the flow interacting element of the operative flow affecting unit is activated and deactivated is predetermined. In another variant the certain time pattern according to which the flow interacting element of the operative flow affecting unit is activated and deactivated is determined during the device's operation.
Optionally, the above device includes at least one sensor for sensing one or more parameters and generating measured data indicative thereof, the one or more parameters comprising at least one of the following: at least one parameter relating to said motion of the main body, at least one parameter relating to the fluid flow, at least one property of the fluid; and a processing utility configured and operable for receiving and processing the measured data from the at least one sensor and determining the time pattern data according to which the at least one flow interacting element of the operative flow affecting unit is activated and deactivated. The parameter relating to said motion may include at least one of an acceleration and a velocity of said main body. The parameter relating to the fluid flow may include at least a fluid flow velocity. The fluid property parameter may include at least one of fluid density and temperature.
Optionally, the oscillator assembly is configured as at least one of the following forms: a spring-based oscillator, a pendulum, an inverted pendulum.
In a variant, the operative flow affecting unit comprises at least one actuator for manipulating the at least one flow interacting element. In another variant, the flow interacting element has one of the following configurations: a retractable flap, a retractable protrusion, a slot. In yet another variant, at least part of the surface of the main body is electrically conductive, and wherein the operative flow affecting unit comprises an electrode insulated from the main body's surface, and the actuator is configured for applying a voltage between the main body's surface and the actuator, thereby creating and accelerating plasma along the surface of the main body and affecting the separation of streams of the fluid flowing over the surface of the main body.
Another aspect of some embodiments of the present invention relates to a method for use in extracting energy from an incoming fluid flow, the method comprising: providing an oscillator assembly mounted on a base, the oscillator assembly comprising: a main body for exposing to an incoming fluid flow; and a joining element attached to the main body and configured for anchoring the main body to the base and enabling oscillation of the main body with respect to said base; subjecting the oscillator to a fluid flow; controlling a separation of the fluid flowing along a surface of the main body by activating and deactivating at least one flow interactive element located on the surface of the main body according to a certain time pattern, the time pattern being selected such that interaction between the flow interacting element and fluid streams creates vortices in the fluid streams at a selected vortex formation frequency causing an increase in oscillation of the main body, thereby enabling conversion of motion from the oscillation into useful energy.
The method may further include converting the motion of the oscillation into useful energy.
Optionally, controlling a separation of the fluid flowing along a surface of the main body comprises increasing the separation between the main body's surface and the streams of fluid flowing over the main body's surface. Additionally or alternatively, controlling a separation of the fluid flowing along a surface of the main body comprises increasing attachment between the main body's surface and the streams of fluid flowing over the main body's surface.
In a variant, controlling a separation of the fluid flowing along a surface of the main body comprises providing the vortex formation frequency approaching a natural frequency of the oscillator.
In another variant, the certain time pattern according to which the flow interacting element of the operative flow affecting unit is activated and deactivated is predetermined. In yet another variant, the certain time pattern according to which the flow interacting element of the operative flow affecting unit is activated and deactivated is determined during the device's operation.
The oscillator assembly may be configured as at least one of the following forms: a spring-based oscillator, a pendulum, an inverted pendulum.
A further aspect of some embodiments of the present invention relates to a system for extracting energy for a fluid flow, comprising: at least one oscillating device configured as the device defined above; at least one compressor, each compressor being powered by the oscillating device and being configured for compressing a working fluid; an accumulation pipe for receiving the compressed working fluid from the at least one compressor and leading the compressed working fluid to a predetermined location, thereby enabling direct use or storage of the compressed working fluid for further use.
Optionally, the system further includes a motor configured for drawing the compressed working fluid located in said predetermined location and using the compressed working fluid to generate useful energy, and venting the working fluid after use.
In a variant, the working fluid is air, and the compressor comprises an air intake for drawing the air.
In a variant, the system further includes a hydraulic return pipe for returning the used working fluid to the at least one compressor.